limited community that spoke Russian. Acquisition theories Heritage language acquisition theories are highly contested. The most common theory is the Incomplete Acquisition Theory, but other scholars have considered delayed acquisition, variations in input, and cross-linguistic influence as factors that contribute to heritage language speakers' competence. For children who may have acquired a different dialect of the heritage language, they would require a unique type of instruction that may differ from styles of instruction that would be most beneficial for students who experience incomplete acquisition at an early age. Acquisition theories have been proposed in an attempt to explain why heritage speakers’ competence in the heritage language diverges from that of their monolingual peers. Silvina Montrul's research has been foundational to this work. Her Incomplete Acquisition Theory states that heritage speakers do not completely acquire the heritage language before switching to the L2. As attrition can also render a grammar “incomplete,” attrition is included in this theory. Multiple researchers have refuted Silvina Montrul's proposal by suggesting that the Incomplete Acquisition Theory does not take into consideration delayed acquisition, input variation, and cross-linguistic influence, all of which factor into language competence. As research on heritage language learners' acquisition is relatively recent, dating back to the early 2000s, there remains much to be discovered about the process. The following acquisition theories pertain to language learners who have learned, at least partially, the heritage language before switching to the dominant language. For more information on heritage language learners who lack personal experience with the heritage language but have ancestors who spoke the language, see language revitalization. Incomplete Acquisition Theory Incomplete Acquisition The Incomplete Acquisition Theory recognizes that complete acquisition of L1 takes place throughout childhood, stretching into the school-age period. Therefore, heritage language learners, who typically switch to the dominant culture's language when they enter school and, consequently, experience a decline in input in the heritage language, do not completely acquire their first language. As heritage language learners are rarely schooled in their L1, they are often orally proficient but illiterate or have underdeveloped written comprehension and production in the heritage language. In treating heritage language learning as interrupted L1 acquisition, in which the learner has access to Universal Grammar, it is expected that heritage language learners have knowledge of concepts found in early stages of L1 development, such as binding constraints, wh-movement, and aspects of lexical semantics, and struggle with grammatical concepts that